A variety of aluminum-alloy plates have been conventionally in use for general purposes as vehicle members of an automobile, a ship, and an aircraft, respectively, in accordance to each property thereof on an alloy-by-alloy basis. In particular, enhancement in fuel economy, through reduction in weight of the vehicle member, is required of the aluminum-alloy plate due to public awareness of recent geoenvironmental issues such as CO2 emission, etc., and there has been an increase in the use of an aluminum-alloy plate whose specific gravity is about one third as compared with iron, while having excellent energy-absorption properties.
Further, in the case of using aluminum alloy as an automobile panel, characteristics of the aluminum alloy, such as formability, weldability, adhesiveness, chemical-conversion properties, corrosion resistance after coating, and attractive appearance, etc., are required. A method of manufacturing an automobile panel with the use of an aluminum alloy includes the steps of 1) forming (cutting into a predetermined size, and press-forming into a predetermined shape, 2) bonding (welding and/or adhesion, 3) chemical-conversion coating (degreasing by use of a cleaning agent→surface adjustment by use of colloidal titanate, etc.→zinc phosphate treatment, and 4) coating (priming through electrodeposition coating→intercoating→finish coating). The method described as above is fundamentally the same as the method adopted in the case of using a conventional steel plate.
Meanwhile, as progress is being made in the trend of automobile parts being assembled into a module, and there has been a tendency that a time interval between the manufacturing of an aluminum alloy plate itself and the automobile panel entering into a body-manufacturing process becomes longer. The trend of automobile parts being assembled into the module is represents a method whereby individual parts fitted to a body directly by an automobile manufacturer are sub-assembled by a parts-supply company in advance before fitted to the body. It is a main object of this method to simplify hard works performed at the automobile manufacturer in the past to thereby improve production efficiency. The method also has favorable effects of shortening a production process, and reduction in work-in-process. Although a burden on the part of the parts-supply company will increase, this method is effective in cost reduction for the automobile manufacturer and the parts-supply company as a whole to thereby contribute to reduction in the production cost of an automobile.
Further, a system of direct delivery from a light-pressure maker (a rolled-product maker) to the automobile manufacturer has thus far been in the mainstream of a transfer channel of an automotive aluminum-alloy plate. However, as further progress is made in the trend of automobile parts being assembled into the module, the transfer channel is inevitably altered to one via the parts-supply company, so that the time interval between the manufacturing of the aluminum alloy plate itself and the automobile parts entering into the body-manufacturing process becomes unavoidably rendered longer than before.
In such a case, however, the surface properties of an automotive aluminum-alloy plate will undergo aged deterioration to adversely affect adhesiveness, chemical-conversion properties, and coating properties, thereby posing a problem. It is known that at the time of chemical-conversion accompanying the aged deterioration, in particular, stability in wettability after degreasing will deteriorate, and a chemical-conversion coating film is rendered hard to adhere, so that corrosion resistance is affected thereby.
For this reason, efforts have thus far been focused on enhancement in the chemical-conversion properties, etc. by removal of magnesium (magnesium is also referred to as “Mg” hereinafter) in the surface of an aluminum-alloy containing magnesium (refer to Patent Documents 1 through 4).
Further, in order to obtain an aluminum-alloy plate excellent in both wettability after degreasing, and adhesiveness, in particular, there has been proposed an aluminum-alloy plate for use as the body sheet of an automobile (refer to Patent Documents 5 and 6), obtained by pouring a pH-adjusted hot water at a temperature 40° C. or higher after annealing to thereby adjust both Mg content and OH content or both Mg content and Si content of the surface film of the aluminum-alloy plate before applying a rust-prevention oil thereto within 14 days after surface adjustment (refer to Patent documents 5, 6).
Still further, in order to obtain an aluminum-alloy plate less in aged deterioration of the characteristics thereof, there has been proposed an aluminum-alloy plate for use in an automobile body, the aluminum-alloy plate being equipped with a metallic aluminum substrate of an aluminum-alloy plate containing 2 to 10 wt % of Mg, a phosphate film of aluminum, formed on the substrate, and an aluminum oxide film formed on the phosphate film (refer to Patent document 7).
Furthermore, in order to obtain an aluminum-alloy plate excellent in degreasing properties, chemical-conversion properties, and adhesiveness, there has been proposed a surface treated aluminum-alloy plate equipped with a film formed on the surface thereof, the film containing a predetermined content of Mg, and Zr, respectively, while containing neither halogen nor phosphorus (refer to Patent document 8).